Correct Option (d):
Zirconium is predominantly non-radioactive in its natural state. It primarily consists of stable isotopes such as Zirconium-90, Zirconium-91, Zirconium-92, and Zirconium-94. Although Zirconium-94 and Zirconium-96 exhibit very weak radioactivity, their extremely long half-lives and low natural abundance render natural zirconium effectively non-radioactive. This property, combined with its low neutron absorption cross-section, makes it suitable for structural applications, including in nuclear reactors.
Incorrect Options:
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Astatine: All isotopes of Astatine are radioactive and unstable. It is one of the rarest elements found naturally on Earth.
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Francium: This alkali metal is highly radioactive. All known isotopes of Francium are unstable and possess very short half-lives.
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Tritium: It is a radioactive isotope of hydrogen (³H). Tritium undergoes beta decay with a half-life of approximately 12.3 years. It finds applications in fusion research and self-luminous devices.